1. Field of the Invention and Related Art
The present invention relates to a toner used for developers for developing electrostatic images in electrophotography, electrostatic recording, electrostatic printing and the like. Particularly, the present invention relates to a positive chargeable magnetic toner which is positively charged in a direct or indirect electrophotographic developing method so as to visualize negative electrostatic images or visualize positive electrostatic images by reversal and, particularly, which exhibits good characteristics in a heated roll fixing method.
Examples of known electrophotographic methods include the methods disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publication Nos. 42-23910 and 43-24748. In general electrophotographic methods, an electrically latent image is formed on a photosensitive material by various means using photoconductive substances and then developed by using a toner to form a toner image which is transferred to a transfer material such as paper or the like as occasion demands and then fixed by heating, pressure, heating/pressure or solvent vapor to produce a copy.
Metals such as amorphous selenium, amorphous silicon and the like, and inorganic compounds such as zinc oxide, cadmium sulfide and the like are generally used as photosensitive materials. However, in recent years, organic photosemiconductors (OPC) containing azo or stilbene dye pigments have been increasingly used in the fields of low-speed machines and popular machines because of their good processability, low price and high safety. It is believed that such photosemiconductors were developed for the field of high-speed machines in anticipation that the durability and sensitivity of such machines would be improved.
Such organic photosemiconductors mainly comprise a photocarrier generating layer and a carrier transporting layer which is provided on the photocarrier generating layer for improving the resistance to wearing, protecting the photocarrier generating layer and improving the sensitivity. In the present situation, since most organic photosemiconductors contain hole transport-type substances and are thus negatively chargeable, electrophotography frequently requires positively chargeable toners.
An effective means for making a toner positively chargeable is to mix as a positive charge controlling agent a compound having an electron donating group in the toner. Examples of positive charge controlling agents that are frequently used include quaternary ammonium salts and nitrogen-containing organic dyes, and basic dyes and salts thereof. Examples of such substances include nigrosine dyes and triphenylmethane dyes. The positively chargeable controlling agents are generally used after they have been added to binder resins and then adjusted to appropriate grain sizes through a heat melting and kneading process, a grinding process and, if required, a classifying process.
However, the charge controlling agents that are generally used easily produce the phenomenon that the charge controllability is changed by a mechanical impact, friction and changes in environmental conditions such as temperature and humidity. If a toner containing a charge controlling agent of the type that is generally utilized is used for development in a copying machine, the toner sometimes deteriorates as the number of times of copying increases.
Particularly, when a dye-type charge controlling agent is used, the dye migrates from the inside of the toner to a charge applying member such as carriers or a development sleeve by virtue of friction, heat or pressure. This frequently causes difficulties in charging the toner, the occurrence of scattering of the toner and deterioration in quality of the image formed.
The positively chargeable magnetic toner also has difficulties in fixing an image.
Methods of fixing a toner image include the following various methods:
(1) a method of fixing an image to a material to which an image is fixed (referred to as "a fixing material" hereinafter) by heating and melting a toner using the heat radiated from a heater; PA0 (2) a method of fixing to a fixing material by applying a pressure to a toner to fluidize it; PA0 (3) a method of fixing to a fixing material by applying solvent vapor to a toner to melt it; and PA0 (4) a method of fixing to a fixing material by applying heat and a pressure to a toner to melt and fluidize it.
The fixing method (1) has a danger of clogging a fixing portion with the fixing material and producing a fire caused by the clogging. The fixing method (2) is not universal because a material for the toner is significantly limited. The fixing method (3) causes the contamination of the surrounding environment. Thus, the fixing method (4) is widely used in the present condition.
The heat/pressure fixing method (4) which uses a heated roll pressed on a toner image has the many advantages that heat can be effectively transmitted to the toner even by a heat source at a relatively low temperature, that the temperature can be easily controlled, and that the deformation of the toner is accelerated by applying a pressure at the same time as heat. However, the fixing method (4) has the following problems to be solved:
In the fixing method (4), since the surface of the heating pressure means such as the heated roller contacts with the toner image in a melted state under pressure, the method produces a so-called offset phenomenon in that the toner image is partially transferred to the surface of the fixing roller and retransferred to a subsequent fixing material, thereby staining the image. In the extreme case, the method produces a so-called winding phenomenon that the fixing material adheres to the heated roll, thereby breaking the fixing unit. In order to prevent the occurrence of this phenomenon, a separating claw for fixing is provided on a heated roll fixing unit so as to force the fixing material to separate from the heated roll. However, this method has the problem that, when the substance is strongly wound on the heated roll, the image on the material is damaged.
In the heat/pressure fixing method, it is therefore necessary that the adhesion of the toner to the heating pressure means is as small as possible.
Thus the surface of the fixing roll is generally made of a material such as silicone rubber, a fluorine resin or the like, which has excellent release properties and low surface energy, and is further coated with a liquid lubricant such as silicone oil or the like, which has good release properties. The effect of silicone oil is very effective for preventing the offset of the toner and the winding of the fixing material. However, the silicone oil has problems in that the use of the silicone oil causes the complication of the fixing unit and the occurrence of an oil stain on the fixing material and that the silicone oil is evaporated by heat and soils the inside of the machine, particularly a charged wire. There is thus a tendency to use no silicone oil or at least to limit the use of the silicone oil.
In order to improve the anti-offset properties of the toner itself, it is effective to restrain the elongation of the toner by increasing the elasticity of the toner in a melted state. It is effective for increasing the elasticity of the toner to add a rubber substance or form a gel by crosslinking.
In order to improve the release properties of the toner itself, it is effective to add wax to the toner. For example, polyolefin wax such as a low-molecular weight polyethylene, low-molecular weight polyproplyene or the like, paraffin wax, wax composed of a long-chain fatty acid ester, amide wax or the like can be employed.
However, the above-described methods are ineffective for the positively chargeable toner and sometimes have adverse effects thereon. For example, a low-surface energy substance such as silicone rubber, fluorine resin or wax has the tendency that the negative chargeability is increased when the substance is charged. Therefore, when such a substance is used in the fixing roll, a so-called electrostatic offset phenomenon readily takes place in which the toner positively charged on the fixing material is electrostatically offset to the fixing roll. Since such a low-surface energy substance originally has low dispersibility and compatibility, when the low-surface energy substance is contained as a surface lubricant in the toner, the surface lubricant easily forms individual particles which are negatively charged to form electrostatic coagula with the positively chargeable toner, thereby producing black point stains on the image. The basic dye or quaternary ammonium salt contained as a positively chargeable controlling agent in the positively chargeable toner soils the silicone rubber of the fixing roll or reacts with the fluorine in the fluorine resin to cause the fixing roll to lose its release properties, elasticity and strength. There are many proposals for solving the above-described problems with respect to the development properties peculiar to the positively chargeable toner and with respect to fixing.
Japanese Patent Laid-Open Nos. 55-134861 and 56-65417 disclose toners each formed by using a binder resin having an acid value. In these documents, the intermolecular cohesion of the binder contained in the toner is increased by introducing an acid value in a binder resin chain so that the adhesive strength between the toner and the fixing paper is increased, and the interfacial tension between the toner and the fixing roll surface is increased. It is recognized that this method is effective for offset and blocking.
Japanese Patent Laid-Open Nos. 57-40264 and 62-21169 disclose combinations of a binder resin containing a thermoplastic resin having an acid group and a positively chargeable controlling agent. In these documents, the toner is stably positively charged by the ionic stability between the base of the charge controlling agent and the acid of the binder resin. However, this method has the problem that, when an acid value is introduced into the positively chargeable toner, the development properties and durability deteriorate. The negative chargeability is increased as the number of the acid value introduced into the toner binder is increased, resulting in the occurrence of a positive fog and a decrease in the image density during preservation. Thus this method cannot be easily effectively used.
Japanese Patent Laid-Open Nos. 57-119364 and 62-9358 disclose quaternary ammonium sulfate compounds and quaternary ammonium sulfonate compounds. In these documents, the reaction of a fixing roller comprising a vinylidene fluoride-hexafluoropropylene copolymer with a quaternary ammonium salt is controlled so as to prevent the deterioration of the heated roll. However, the quaternary ammonium salt functions as a weak positively chargeable controlling agent. When the toner is mixed with carrier particles having strong negative chargeability, the toner produces a tribo-charge sufficient for practical use, while when one-component toner is used without being mixed with carrier particles, sufficient contact between the toner and the fixing material cannot be easily provided, as compared with the case where the toner is mixed with carrier particles and used. There is thus the disadvantage that since the toner is insufficiently charged, the image density is decreased, and fogging is increased. Particularly, under environmental conditions of high temperature and high humidity, the image density is further decreased.
Attempts have been made to improve the controllability of a charge controlling agent by combining a quaternary ammonium salt with another positively chargeable controlling agent. For example, Japanese Patent Laid-Open No. 61-172155 discloses the combination of a quaternary ammonium salt and nigrosine or a positively chargeable thermoplastic resin. In this case, since the quaternary ammonium salt has only an auxiliary function, when a one-component magnetic toner which is not mixed with carrier particles is used, the amount of the other charge controlling component combined with the quaternary ammonium salt must be substantially equal to the required amount of the other charge controlling agent when it is singly used. In this case, there remains the danger of contaminating the charge applying member with nigrosine.
Japanese Patent Laid-Open Nos. 55-113054, 61-80261 and 62-9358 disclose toners each comprising a styrene-butadiene copolymer and a positively chargeable controlling agent. The introduction of a conjugated diolefin such as butadiene or the like into a molecular chain provides the toner with rubber elasticity and is thus effective for anti-offset properties and anti-winding properties. When a positively chargeable toner is used, such a conjugated diolefin has no hindrance but has the advantage that the toner tribo-charge rapidly rises. While when the toner is combined with a positively chargeable controlling agent and a conjugated diolefin, the toner tends to be excessively charged and has the problem with respect to its poor compatibility with other resins and chargeable controlling agents. Particularly, when the toner is used as a one-component magnetic toner without mixing with carrier particles, the magnetic substance has poor dispersibility, and thus fogging sometimes occurs.